This invention relates to intrusion detection systems, and more particularly to systems for detecting the presence of an intruder within the boundaries of an area under surveillance.
Numerous systems have been designed and are presently in use which use pyroelectric or other heat sensitive materials as intruder sensing elements. Pyroelelectric materials include plastic film materials such as polyvinylidene fluoride, crystal materials such as lithium tantalate, and ceramic materials such as lead zirconate titanate. Such devices typically are poled, i.e., polarized, and have electrodes on their polarized areas such that, when radiant infrared energy falls upon the material, a small voltage appears between the electrodes due to internal transfer of electric charge that is amplified to signal an intrusion. Each sensor element is adapted to view one or more different areas in the space under surveillance (by means of focusing lenses or mirrors, for example). When an intruder enters one of the fields of view, the intruder's body heat causes a momentary change in the temperature of that sensor element which causes an output voltage to be produced across its load impedance. This voltage is amplified and an alarm signal is generated in response thereto.
Because these pyroelectric materials are extremely sensitive to temperature (and usually to pressure), the devices respond to environmental changes in pressure and temperature. In an effort to reduce alarms generated by such environmental changes, sensitive areas (elements) have been connected in electrical series or parallel opposition for common mode rejection. In response to an environmental change, both elements are excited equally and because they are connected in electrical opposition, the output is cancelled and no alarm is generated. Such systems also tend to produce occasional output voltage artifacts in the form of "bursts" and/or spikes (due to defects in the elements or in the amplifiers) which cause false alarms.